The present invention generally relates to avionics systems in commercial and military aircraft, and more specifically, to the control of oil cooling and lubrication systems.
Aircraft turbine engines require cooling and lubrication systems to maintain a flow of oil through the engine. Engines in general typically rely on the force of gravity to maintain the lubricating fluid, typically oil, in a reservoir providing oil for pumps moving the oil to wetted components in the machine. If the oil is in a reservoir in which the oil may collect in a known portion of the reservoir, then inlets, valving, and conduits may be reliably placed so as to the maintain the oil flow throughout the engine.
However, aircraft turbine engines, regardless of whether they are involved with propulsive systems or non-propulsive systems such as auxiliary power or environmental cabin systems, cannot rely on the force of gravity in this manner, since they must operate through extreme attitudes in which gravity does not always operate in the same direction with respect to the engine. Furthermore, varying acceleration forces are superimposed upon the force of gravity so that the oil contained in the lubrication system may be subjected to forces coming from any direction. These acceleration forces may cause the lubricating oil to be positioned at any portion of the tank, so that an inlet advantageously located for a gravity fed system may be starved for lubricating oil and thus cause a lower or inadequate oil pressure in the system. This imposes special requirements upon the design and configuration of such lubrication systems in order to maintain continuous and sufficient flow of lubrication to high speed, gas turbine engines typically found in aircraft. Not only must the physical components of the aircraft lubrication systems be designed to accommodate acceleration forces coming from any direction, the components must also be synchronously controlled so that they cooperatively move the lubricating fluid through the system when the system is subjected to varying attitudes or acceleration forces.
These lubricating systems must also be capable of reacting to changes in aircraft operating mode. For propulsive turbine machines, a number of methods have been devised to maintain the flow of lubricating oil to the engine during variable acceleration forces. However, for non-propulsive turbine machines, these methods have been found to be overly complex and thus inappropriate with respect to the less critical nature of non-propulsive turbine machines. For example, if the aircraft is in takeoff mode or combat (surge) mode, high engine speeds are required which necessitate increased lubricating oil flow for cooling the propulsive turbine machine, but the non-propulsive turbine machines do not have such critical cooling and lubrication requirements and can sustain short periods of little or not lubrication without damage. Such cooling requirements may have only a minimal effect on oil pressure for non-propulsive turbine machine, and necessitate the use of additional controls that respond to oil temperature.
Simple volumetric pumps are sometimes used, which are operated by a shaft that is driven by the turbine engine, so that the speed of the pump is directly proportional to the speed of the engine. Thus, the shaft speed at which such pumps operate is not adjustable independently of engine speed and may not be responsive to the actual lubrication needs of the engine.
Finally, turbine machines used for propulsion have different operating parameters than turbine machines used for auxiliary tasks, such as auxiliary power units and environmental systems. These latter non-propulsive turbine machines do not necessarily require continuous, non-interruptible lubrication and can continue to operate for as long as 30 seconds during complete oil deprivation, or longer at reduced oil flow rates, before the oil wetted component is subjected to damaging distress.
The prior art contains numerous examples of how these control problems have been addressed in aircraft engine lubrication systems. U.S. Pat. No. 6,463,819 to Rago discloses a unitary valve that provides an uninterrupted oil supply during different flight attitudes. An oil reservoir is provided having multiple outlet ports to accommodate an oil supply that may be in different parts of the reservoir. A unitary valve senses the oil pressure in a journal enclosure that maintains lubricating oil around a turbine shaft. This change in oil pressure may result from oil starvation at the oil pump when the aircraft changes attitude. The unitary valve reacts hydraulically to the change in oil pressure and directs the input to the oil pump to an alternate outlet port where the lubricating oil might be oriented within the oil reservoir. This arrangement reacts strictly to oil pressure and may not be able to provide sufficient oil in the event oil temperature increases, necessitating an increased flow of lubricating oil to cool turbine engine parts.
U.S. Pat. No. 5,152,141 to Rumford proposes a process for electrically driving and managing the oil pump of a gas turbine engine. Electrical power is supplied to one or more electronic controllers which manage a plurality of electric motors that are typically coupled to oil pumps, among other equipment. According to the U.S. Pat. No. 5,152,141, after starting the main engine, the electronic controller is used to increase the speed of the auxiliary electric motors until they are synchronous with a starter-generator, the rotational speed of which is continuously proportional to that of the turbine engine. Next, the functioning of the auxiliary electric motors continues while maintaining electrical coupling between the starter-generator and each of these motors. In particular, the speed of the oil pump varies between start-up and the maximum speed of the engine along a predetermined acceleration curve. The acceleration communicated to this pump is chosen to allow optimum lubrication of the moving parts of the turbine engine. However the shaft speed of the electric motors is not responsive to the attitude of the aircraft and may not provide the amount of oil necessary for different maneuvers.
U.S. Pat. Appl. Pub. No. US2001/0047647 to Cornet discloses a process for lubricating an aircraft engine. The process employs a variable speed pump that operates independently of the rotational speed of the engine shaft and that is controlled by a control system preferably in the form of a predetermined law in order to adapt to the actual lubrication needs of the engine. The laws may be according to open-loop, closed-loop, or fuzzy logic, each based on one or more engine parameters such as pressure, temperature, shaft speed, and mechanical load. However, Pat. Appl. Pub. US2001/0047647 does not address the issues of using the control logic as a function of inverted flight, airframe maneuvers, or non-gravitational accelerations.
As can be seen, there is a need for a method of controlling a lubrication circuit, where the method is simple, straightforward, and responsive to changing turbine machine needs and to airframe maneuver parameters, so that the lubrication circuit can provide sufficient lubrication regardless of airframe maneuver forces.